Typical brake pistons in a disc brake reside within one or more piston bores of a caliper. The brake pistons generally have a cylindrical shape with a closed end opposing an open end. The closed end is in contact with brake fluid and the open end is usually in contact with a brake pad. The vehicle typically has a master cylinder in connection with both a brake pedal and the brake piston. The master cylinder generally converts non-hydraulic pressure (i.e., depression of a vehicle brake pedal) into hydraulic pressure of the brake fluid acting on the brake piston. Depressing of a vehicle brake pedal results in brake fluid flowing from the master cylinder to the caliper which then results in increased pressure of the brake fluid applying a brake apply force to the closed end of the brake piston. The force on the brake piston from the brake fluid includes both axial force on the piston and radial force on the piston. The open end of the brake piston then pushes the brake pad against the brake rotor to transfer the brake apply force to the rotor and result in braking of the vehicle. Examples of such pistons are disclosed in U.S. Pat. Nos. 4,193,179; 6,637,317; and 7,000,526 all of which are expressly incorporated herein by reference for all purposes.
Historically, brake pistons were made of steel, so as to be sufficiently rigid to withstand the axial and radial forces of the brake fluid, the counter force of the brake pad during brake apply, and prevent degradation of the brake fluid. Metallic brake pistons have a high thermal conductivity, which transfers heat from the brake pads into the brake fluid, and during braking the brake fluid becomes hot and may boil. The heated brake fluid may result in a soft brake pedal feel as the brake fluid may become more compressible and a vehicle driver may need to depress the brake pedal a further distance to apply a braking force or the brake system may fail as no amount of brake pedal depression is able to apply sufficient hydraulic pressure to result in the brake pad stopping movement of the brake rotor.
To provide for improved heat dissipation and corrosion resistance, brake pistons have been manufactured from polymers, thermosets, or glass fibers. However, brake pistons made from polymers have a tendency to deflect upon axial forces and/or radial forces being applied. This piston deflection results in an increase in required brake pedal travel (i.e., brake pedal depression distance) to achieve brake clamping force. This increased brake pedal travel is dependent on the piston size, the brake system, the clamping force, and the designed piston stiffness. Piston deflection generally includes a piston axial reduction (i.e., reduction in brake piston's overall length) from about 0.03 mm to about 0.06 mm and a radial reduction (i.e., reduction in brake piston's outer diameter) from about 0.006 mm to about 0.012 mm when receiving from about 0 MPa to about 15 MPa of pressure from the brake fluid. Due to the brake piston deflection, a vehicle driver may feel reduced brake stiffness and/or a looser pedal feel as the brake piston deflects. Because of this reduced stiffness and looser pedal feel, a vehicle driver may perceive a lower quality braking system, and even vehicle. In order to achieve a tighter pedal feel with a standard piston, less running clearance will be available between the brake pads and the rotor. Because of this lower running clearance, the brake piston may push the brake pad toward the rotor and apply an off-brake drag force. The off-brake drag force results in a reduced brake pad life due to the increased rotor contact and an increase in fuel consumption by a vehicle due to having to overcome the off-brake drag force.
Additionally, for vehicles having sport-tuned suspensions or other high performance and/or luxury vehicles, consumers desire a tight pedal feel with increased stiffness. This tactile feeling has generally only been achievable through completely metallic brake pistons, polymeric brake pistons with increased material thickness, or changing the material of the brake pad.
Exemplary plastic brake pistons are discussed in U.S. Pat. Nos. 4,449,447; 8,348,030, 6,085,636; and 5,845,747, which are expressly incorporated herein by reference for all purposes. Notwithstanding the above, there appears an absence in teaching of how to reinforce a polymer based brake piston with a metallic insert while preventing heat transfer from the friction material to the brake fluid.
Thus, what is needed is a brake piston able to provide for improved heat dissipation, able to prevent overheating of brake fluid, while also having decreased deflection during application of a braking force. What is needed is a brake piston capable of providing a tight pedal feel with increased stiffness without having to be comprised completely of metal, without having to increase the thickness of a polymeric piston, and allowing the use of standard brake pad material. What is needed is a brake piston having an increased stiffness to increase running clearance and lower off-brake drag, improving fuel efficiency and brake pad life.